Variable time ratio controls are known for D.C. traction motors energized from a battery wherein a semiconductor power switch is switched rapidly between the "on" state and the "off" state to selectively vary the average voltage applied to the motor and thus regulate its speed. Such time ratio controls may be of the pulse width modulation type wherein variable width constant frequency pulses are applied to the motor in order to vary the duty cycle of the power switch, or may be of the frequency modulation type wherein a controlled rectifier power switch is gated on at an adjustable frequency by a relaxation oscillator to vary the average power input to the motor. A free-wheeling diode is commonly connected in shunt to the inductive motor circuit to provide a path for the inductive current when the semiconductor power switch is open and thus prevent abrupt current change and resultant high voltage across the power switch. Such free-wheeling diode shunting the series combination of motor armature and field winding maintains the flow of current to the motor during the interpulse period and reduces motor ripple current. An armature diode is commonly connected in shunt to the motor armature and provides a path for armature current flow during braking.
It is often desirable to dynamically brake, or plug, a D.C. traction motor by reversing the connection to the motor field while it is coasting so that the motor acts as a generator with the polarity reversed while the vehicle is still going forward. Very high magnitudes of current flow in the motor during plugging which may cause jolts and jerking of the vehicle and also shorten motor brush life. Plug sensing circuits are known which detect a plug condition and limit the on-time of the semiconductor power switch in response thereto in an attempt to prevent damage to the brushes and bring the motor to a smooth stop. However, known plug sensing circuits, such as those which detect average armature voltage or are responsive to a predetermined magnitude of current through the armature diode, are relatively slow and inaccurate in sensing and do not respond with sufficient speed to limit on-time of the power switch before high magnitudes of current flow in the motor.